DEVELOPMENT OF SPELERPES BILINEATUS 187 



sional triangular cells being found. At other times, the cells 

 are conspicuously lobed as seen in the yolk cells of figs. 14 and IG. 

 The same thing at a sixteen-cell stage is shown in figs. 20 and 21. 

 In still another type, where no cleavage cavity at all is formed, 

 the microtneres lie directly upon the macromeres. When present, 

 the cleavage cavity may be large as in fig. 14, or quite small as 

 in figs. 20 and 21, where it is represented by intercellular spaces. 



A.S cleavage proceeds, a condition develops similar to that 

 figured by Gronroos ('95) for Salamandra maculosa. There is 

 an outer layer of cells upon the upper hemisphere and lying within 

 this, a second series concentric with the first (figs. 16-17). The 

 cells of the outer layer are smaller, more angular and, for the most 

 part, contain fine yolk-granules, although sometimes their inner 

 ends contain coarse granules. The inner layer has large, rounded 

 cells with coarse yolk granules except that occasionally finer 

 granules occur in their outer ends (fig. 16). This condition arises 

 in the following manner: At the twelve-cell stage there is a 

 rather thin cap of eight cells lying on the surface of the egg (fig. 

 15), which has arisen by division of the four small blastomeres of 

 the eight-celled stage. The remainder of the egg shows no sign 

 of division except those of earlier planes. After the next few 

 divisions, the yolk mass has usually given off one to four large 

 rounded cells, which lie above the center of the egg. By repeated 

 divisions of the cells already formed and the addition of cells 

 thereto from the lateral parts of the yolk, both layers increase in 

 size (figs. 17 and 18). There is often no sharp boundary between 

 the two layers (fig. 17). Sometimes the lower parts of the cells 

 belonging to the outer layer are added to the inner in the manner 

 described by Reed ('05) for the frog. Quite often one or two 

 cells are much larger than the rest of the inner layer. Similar 

 cells or their descendants may often be seen until a comparatively 

 late period (fig. 18). 



During this time, the cleavage cavity has existed for the most 

 part as intercellular spaces, although sometimes it is well devel- 

 oped. Then a well-defined outer layer appears (fig. 18). The 

 next step, presumably, is that those yolk cells which do not 

 enter into the formation of the upper layer of cells, sink down. 



